Dot matrix character printers are well known in the art, particularly for use as output devices for computers and other electronic devices. These printers are used to produce printed character output on a paper medium in response to receiving electronic signals which correspond to the characters of the available character sets. Individual characters are formed from an appropriate combination of dots selected from an array of available dot locations. Generally, these dot locations are arranged in an n-column by m-row matrix. A printing head contains a solenoid-operated print rod for each row location in the matrix. When the printing of a character requires that a dot be written in a particular row, the corresponding solenoid in the printing head is activated; this causes the associated printing rod to be flung (or "fired") in the direction of the paper (i.e., "target"). An inked printing ribbon is interposed between the printing head and the paper. Thus, the fired printing rod impacts the ribbon and prints an impression of its end surface on the paper. The printing rod is actually a thin wire-shaped element with a flat end having a round cross section. It is this end which forms a dot on the paper.
Typically, the printing head of a dot matrix printer contains printing solenoids. These are assembled into close proximity with each other. The activation of these printing solenoids causes them to heat up. Dissipation of solenoid heat is normally not a problem in slow speed character printers. However, it has been observed to sometimes become a problem in high speed dot matrix character printers. When the print head is activated for a substantial time and the characters to be generated in a relatively short interval require the printing of a somewhat higher than average number of dots during that interval, solenoid heat may be inadequately dissipated. When that happens, solenoid response characteristics may be altered due to parametric variations, and one or more of the printing rods may cause a dot to be printed slightly earlier or later than intended, or even not at all. This degrades the appearance of dots and characters printed under such conditions and may, under severe conditions, even lead to the printing of unrecognizable or incorrect characters. Solenoid power supply failure or degraded performance may also result, as a greater load is imposed thereon. And excessive heat may cause permanent damage to the solenoids or other components. Considerable efficiency is lost if the character printing rate is significantly reduced in order to avoid the solenoid heat dissipation problem, since high reliability would require a substantial reduction in speed.
Typically, in the prior art, in order to change the pitch of printed characters, it has been necessary to provide additional tracks on the encoder at different pitches or to physically change the encoder-to-carriage "gear" ratio. These approaches have typically been cumbersome and not conducive to changing pitch on a character to character basis.
It should also be noted that the printing heads of prior art printers generally have their print rods all arrayed in a single vertical column. Unless the print head is capable of vertical motion, this means that dots formed by adjacent print wires (or rods) cannot overlap; some dot spread caused by the ribbon may, however, allow some blending together of adjacent dots, but resolution is ultimately limited to that provided by tangentially touching dots.